Coated abrasives typically consist of a backing substrate, abrasive grains, and a bonding system which operates to hold the abrasive grains to the backing. In a typical coated abrasive product, the backing is first coated with a layer of adhesive, commonly referred to as a "make coat", and then the abrasive grains are applied. The resulting adhesive/abrasive composite layer is then generally solidified or set enough to retain the abrasive grains to the backing, so that a second layer of adhesive, commonly referred to as a "size coat", can be applied. The size coat further reinforces the coated abrasive product upon setting. Optionally, a "supersize coat", which may contain grinding aids, can be applied over the solidified size coat. Once the size coat and supesize coat, if used, has cured, the resulting coated abrasive product can be formed into a variety of convenient articles such as sheets, rolls, belts and discs.
The backing substrate used in coated abrasive products is typically chosen from paper, polymeric film, cloth, vulcanized fiber, nonwoven web, combinations thereof, or treated versions of these. Commonly used abrasive grains include flint, garnet, emery, silicon carbide, fused aluminum oxide, ceramic aluminum oxide, fused alumina-zirconia, fused zirconia, diamonds, multi-grain granules. Conventional bond systems typically comprise a glutinous or resinous adhesive such as hide glue, phenolic, epoxy, acrylate, melamine, urethane, ureaformaldehyde or mixtures thereof. Fillers are sometimes added to the adhesive to reduce the cost and to improve the cured resin's heat stability and hardness.
For many years fused aluminum oxide and silicon carbide were the primary abrasive grains used in coated abrasives. This has been changed somewhat by the development of "premium" abrasive grains such as fused alumina-zirconia (commercially available from the Norton Company of Worcester, Mass. under the trade designation NorZon) and alpha alumina-based ceramic materials (commercially available from the 3M Company of St. Paul, Minn. under the trade designation Cubitron.RTM.). Coated abrasive products containing these premium abrasive grains generally perform better in stock removal applications than coated abrasive products containing fused aluminum oxide or silicon carbide. Fused alumina-zirconia and alpha alumina-based ceramic materials are not universally used in coated abrasives, however, due to their high cost in comparison to fused aluminum oxide and silicon carbide. Thus, an incentive exists to reduce the cost of coated abrasive products containing these premium abrasives without sacrificing their performance.
The present invention achieves this goal by using premium abrasive grains in combination with nonabrasive inorganic diluent grains whose Knoop hardness is less than 200. The addition of the diluent grain provides a coated abrasive product of lower cost having equal or improved performance when compared to a coated abrasive product comprised only of the premium abrasive grains.
The blending of two or more types of grains to reduce the cost and/or to improve the performance of an abrasive article is well known in the art. Examples disclosing such blending include:
U.S. Pat. No. 2,410,506, Kirchner et al, which discloses a coated abrasive article wherein an expensive diamond abrasive grit is diluted with relatively inexpensive silicon carbide abrasive grits.
Assignee's European patent application, EP 228,856 published Jul. 15, 1987, which discloses blending abrasive grits formed of ceramic-containing oxides of aluminum and and yttrium with less expensive conventional abrasive grits or materials which are not noted as abrasive, such as marble, glass and the like, in an abrasive product to reduce cost.
U.S. Pat. No. 3,175,894, Foot, which discloses a bonded abrasive article comprised of an admixture of fused alumina abrasive particles and fused zirconia abrasive particles. The combination of the two grains is reported to produce an abrasive article having performance characteristics superior to articles made of either fused alumina or fused zirconia alone. Additionally, diluting the expensive fused zirconia with less expensive fused alumina abrasive grains lowers the cost of the abrasive article.
U.S. Pat. No. 1,830,757, Hartmann, which discloses abrasive articles, both bonded and coated, comprised of a mixture of abrasive particles having a Moh's hardness of 9 or greater and friable nonabrasive particles having a Moh's hardness below 9. The nonabrasive particles reportedly may be any particles of a granular nature that are more friable than the abrasive grains and yet firm enough to break out of the bonded mass without glazing the surface of the article, or that do not form as firm a union with the bond adhesive as do the abrasive particles. During grinding, the friable grains are said to break apart and leave holes or depressions over the grinding face which results in an open, sharp-cutting surface that improves the abrasive action.
U.S. Pat. No. 3,476,537, Markotan, which discloses abrasive articles, both bonded and coated, in which porosity has been induced by the addition, to the abrasive composition, of a granular agent approximating the abrasive grains in size but softer than the abrasive grains. The porosity inducing agent is preferably one that is widely available at very low cost, as compared with that of the abrasive grit material. The porosity inducing agent reportedly may be selected from limestone, natural or activated bauxite, and minerals such as olivine, gypsum, chromite, coquimbite, pyrolusite, molybdenite, galena, halite and the like, as well as a variety of products manufactured for a similar purpose. It is noted that the materials referred to above vary quite widely in hardness on the Moh and Knoop scales, i.e., from Moh's Nos. 1-3 to as high as 6 or 7. These improved abrasive products reportedly will remove more stock than, or at least as much as, a conventional product.
U.S. Pat. No. 3,996,702, Leahy, which discloses a coated abrasive product using fused zirconia as the abrasive. It is considered desirable, however, to include a substantial portion of alumina abrasive grains or other diluent to reduce the cost of the product without unduly reducing performance. The alumina grains can either be blended with zirconia grains or, fused alumina-zirconia grains may be formed by crushing hardened fused blends of alumina and zirconia. If desired, softer grains such as flint, which function in a manner analogous to a filler or diluent, reportedly may be blended with fused zirconia containing grains.
U.S. Pat. No. 3,266,878, Timmer et al., which discloses a coated abrasive product wherein diamond abrasive is diluted with an abrasive material capable of being formed into discrete particles and having a Moh's hardness within the range from 4.0 to 8.5. The dilution of the diamond abrasive reportedly increases the cut of the abrasive surface and reduces the cost of the abrasive article.
Canadian Patent No. 802,150, published Feb. 11, 1964, Cadwell, which discloses a coated abrasive product comprising diamond abrasive granules diluted with granules having a Knoop hardness in the range from 200 to 650. Examples of this invention reportedly removed three times more stock per carat of diamond consumed than conventional diamond articles such as described in Kirchner et al. discussed above.
U.S. Pat. No. 4,734,104, Broberg, and U.S. Pat. No. 4,737,163, Larkey, which disclose coated abrasive products wherein the abrasive grains comprise a mixture of expensive "superior" abrasive grains, such as co-fused alumina-zirconia, and alpha alumina-based ceramic grains, with other abrasive grains such as fused alumina. The superior abrasive grains were concentrated in the coarse fraction of the abrasive grain grading sequence while the other abrasive grains were concentrated in the fine fraction. Reportedly, the addition of the superior abrasive grains improved abrading performance significantly more than would be expected, with products containing the blend of grains performing superiorly, in some cases, to products made with either abrasive grain alone.
It should be clear at this point that Applicant does not content that he has been the first to incorporate nonabrasive inorganic diluent grains having a Knoop hardness less than 200 into an abrasive article. Markotan discloses abrasive articles wherein known abrasives are diluted with granular materials such as limestone and gypsum. Rather, the invention is primarily concerned with the unexpected discovery that blending premium abrasive grains with nonabrasive inorganic diluent grains having a Knoop hardness less than 200, in a coated abrasive product, not only lowers the cost of the article, but does so without reducing the article's performance.
The benefit of a reduced article cost without a corresponding reduction in performance for coated abrasive products containing nonabrasive inorganic diluents selected from the class having a Knoop hardness less than 200 is unexpected in view of the state of the art. The prior art appears to limit the class of diluents capable of providing this benefit in coated abrasive articles to diluents having greater hardness. For example, Cadwell limits the diluents to those having a Knoop hardness in the range from 200 to 650 and Timmer et al. limit the diluents to those having a Moh's hardness from 4.0 to 8.5. Thus, the art teaches away from the use of diluents having a Knoop hardness less than 200 to achieve this benefit in coated abrasives.
Additionally, although coated abrasive products containing diluents within this class are included within the disclosures of Hartmann and Markotan, these two patents deal primarily with bonded abrasive products. In fact, all of the examples disclosed in these two patents showing improved or equal performance by abrasive products containing these diluents used bonded abrasive wheels. However, due to the difference in mode of operation between bonded and coated abrasives, improved or equal performance in coated abrasive articles would not be expected to follow directly from the improved performance attributable to the incorporation of these diluents in bonded abrasives. Bonded abrasives rely upon the continual breakdown and removal of the abrasive grains on the cutting surface to continually present sharp cutting points to the material being ground. The soft nonabrasive diluents improve the performance of bonded abrasives since they breakdown quickly during the grinding action and leave holes or depressions over the grinding face, which aid the breakdown of the abrasive grains and help maintain a sharp-cutting grinding surface. Coated abrasives, on the other hand, have only a single layer of abrasive grains. Thus, adding soft nonabrasive diluents, which breakdown quickly under grinding action and aid the breakdown of the abrasive grains, to coated abrasives, would be expected to lead to the removal of the entire cutting surface, thereby reducing the life and performance of the abrasive article.